1. Field of the Invention
This invention relates to an image recording apparatus for irradiating a recording medium with a laser beam emitted from a laser source, based on image signals created with reference to a dot clock signal corresponding to a resolution and the like of an image to be recorded.
2. Description of the Related Art
An image recording apparatus, as described in Japan Patent No. 3544593, for example, includes a recording drum having a recording medium mounted peripherally thereof, a rotating mechanism for rotating the recording drum about its axis, and a recording head having a laser source and movable axially of the recording drum. While irradiating the recording medium with a laser beam from the laser source, its irradiation point is moved in a primary scanning direction by rotation of the recording drum, and moved in a secondary scanning direction by movement of the recording head.
When an image is recorded on various types of recording medium using such an image recording apparatus, the image sometimes cannot be recorded properly due to a difference in sensitivity between the recording mediums.
In a CTP (computer-to-plate) apparatus for making a printing plate for flexography, for example, an image may be recorded on both a flexographic plate with low sensitivity and an offset printing plate with high sensitivity. When the image is recorded on the offset printing plate by employing a construction that can record the image on the flexographic plate with low sensitivity, the intensity of the laser beam is too strong for the sensitivity of the plate. The laser beam can irradiate a larger area than is required, or damage the surface of the printing plate. Thus, the image cannot be recorded properly.
To avoid such an inconvenience, it is conceivable to lower the intensity of the laser beam outputted from the laser source by reducing electric current applied to the laser source. However, there is a limit to the intensity of the laser beam reducible by this technique. When the current applied to the laser source is reduced below a certain level, there will arise a problem that the beam profile of the laser beam emitted from the laser source changes.
It is also conceivable, as described in Japan Patent No. 3544593, to lower the intensity of the laser beam by reducing a pulse width when recording dots forming each image. In such a case, however, the area of the dots forming each image becomes small, resulting in a problem that formation of a proper image becomes difficult.